Variable filter-based optical spectrometer

Abstract

An optical spectrometer uses a tapered Fabry-Perot type variable optical filter in conjunction with an optical detector array. The filter can be a long-pass, short-pass, or narrow bandpass filter. The stability of the variable optical filter allows high resolution, depending on the number and spacing of the detectors used. In a further embodiment, signal-processing techniques are used to enhance the resolution of the spectrometer beyond the measured response.

What is claimed is:  
     
       1. An optical spectrometer component comprising: 
       a fiber optic input;  
       a magnifying lens disposed to expand an optical signal from the fiber optic input to  
       a collimating lens, the collimating lens disposed to provide a light beam to  
       a variable bandpass filter having  
       an etalon structure with  
       a tapered spacer region being tapered along a taper direction, the variable bandpass filter having a thermal stability of less than or equal to 50 parts per million per degree Centigrade of ambient temperature change; and  
       a linear optical detector array disposed along the taper direction.  
     
     
       2. The optical spectrometer of  claim 1  wherein the variable bandpass filter has 
       a first reflector comprising a first plurality of high-index layers and a first plurality of SiO 2  layers, the first plurality of high-index layers alternating with the first plurality of SiO 2  layers; and  
       a second reflector comprising a second plurality of high-index layers and a second plurality of SiO 2  layers, the second plurality of high-index layers alternating with the second plurality of SiO 2  layers wherein the tapered spacer region comprises SiO 2 .  
     
     
       3. The optical spectrometer of  claim 2  wherein at least some layers of the first plurality of high-index layers comprise Ta 2 O 5 . 
     
     
       4. The optical spectrometer of  claim 2  wherein at least some layers of the first plurality of high-index layers comprise Nb 2 O 5 . 
     
     
       5. The optical spectrometer of  claim 1  wherein the variable filter has a thermal stability of lees than 25 parts per million per degree Centigrade of ambient temperature change. 
     
     
       6. The optical spectrometer of  claim 1  wherein the variable filter has a thermal stability of less than 10 parts per million per degree Centigrade of ambient temperature change. 
     
     
       7. The optical spectrometer of  claim 1  wherein the optical detector array has a length along the taper direction of less than or equal to 12 mm. 
     
     
       8. The optical spectrometer of  claim 1  wherein the variable bandpass filter has a 50% bandwidth of less than or equal to about 0.6 nm at a center wavelength, the center wavelength being between about 1530-1600 nm. 
     
     
       9. The optical spectrometer component of  claim 1  wherein the variable bandpass filter is a linear variable bandpass filter. 
     
     
       10. An optical spectrometer component comprising: 
       a fiber optic input;  
       magnifying lens disposed to expand an optical signal from the fiber optic input to  
       a collimating lens, the collimating lens disposed to provide a light beam to  
       a variable bandpass filter having  
       an etalon structure with  
       a tapered spacer region being tapered along a taper direction, the variable filter having a thermal stability of less than or equal to 50 parts per million per degree Centigrade of ambient temperature change and a 50% bandwidth of less than or equal to about 0.6 nm at a center wavelength, the center wavelength being between about 1530-1600 nm; and  
       a linear optical detector array disposed along the taper direction, the linear optical detector array having a length of less than or equal to 12 nm along the taper direction.  
     
     
       11. The optical spectrometer component of  claim 10  wherein the linear optical detector array has at least 256 pixels. 
     
     
       12. The optical spectrometer component of  claim 10  wherein the linear optical detector array has at least about 512 pixels so as to provide a nominal resolution of the optical spectrometer component of about 3 Angstroms or less. 
     
     
       13. The optical spectrometer component of  claim 10  wherein the variable bandpass filter is a linear variable bandpass filter. 
     
     
       14. An optical transmission network comprising: 
       an input optical fiber configured to carry a plurality of wavelength-division-multiplexed optical signals having nominal channel spacing of about 200 GHz or less;  
       an output optical fiber;  
       an optical tap disposed between the input optical fiber and the output optical fiber and configured to couple a portion of at least some of the plurality of wavelength-division-multiplexed optical signals to  
       an optical spectrometer component having  
       a variable filter including an etalon structure with at least one tapered spacer region being tapered along a taper direction with a thermal stability of less than 50 parts per million per degree Centigrade of ambient temperature change, and  
       a detector array having n detectors affixed to the variable filter providing a nominal resolution; and  
       an analyzer coupled to the optical spectrometer component so as to monitor each of the some of the plurality of optical signals and having a memory with a calibration array for m calibration wavelengths wherein m is greater than n to provide a spectrometer resolution that is greater than the nominal resolution.  
     
     
       15. The optical transmission network of  claim 14  wherein the variable filter is a linear variable filter. 
     
     
       16. An optical spectrometer comprising: 
       a fiber optic input;  
       collimating optics;  
       a continuous linear variable filter having a tapered spacer region being tapered along a taper direction and having a thermal stability of less than 50 parts per million per degree Centigrade of ambient temperature change;  
       a linear optical detector array having n detectors disposed along the taper direction and providing a nominal resolution;  
       an analyzer electrically coupled to the linear optical detector array including a memory storing a calibration array for m calibration wavelengths wherein m is greater than n to provide a spectrometer resolution that is greater than the nominal resolution and wherein the collimating optics are disposed between the fiber optic input and the linear variable filter to illuminate the liner variable filter with a collimated light beam.  
     
     
       17. The optical spectrometer of  claim 16  wherein the linear variable filter is a linear variable edge filter. 
     
     
       18. An optical transmission network comprising: 
       an input optical fiber configured to carry a plurality of wavelength-division-multiplexed optical signals having nominal channel spacing of about 200 GHz or less;  
       an output optical fiber;  
       an optical tap disposed between the input optical fiber and the output optical fiber and configured to couple a portion of at least some of the plurality of wavelength-division-multiplexed optical signals to  
       an optical spectrometer having a nominal resolution less than or equal to 8 Angstroms and a thermal stability of less than 50 parts per million per degree Centigrade of ambient temperature change and including  
       a variable filter with at least one tapered spacer region being tapered along a taper direction, and  
       a detector array affixed to the variable filter; each  
       an analyzer coupled to the optical spectrometer component so as to monitor each of the some of the plurality of optical signals.  
     
     
       19. The optical transmission network of  claim 18  wherein the optical spectrometer further comprises an analog-to-digital converter to provide digital electronic signals to the analyzer. 
     
     
       20. The optical transmission network of  claim 18  wherein the variable filter is a linear variable filter.